Many types of pipe systems for transporting fluids, such as aggressive fluids, are known in the art, including both onshore and offshore pipe systems, such as pipes for transporting hydrocarbon containing fluids. Often the pipe system is subjected to high and often varying pressures both within its flow channel and if used offshore also from the surrounding water in which it is used or from being trenched and covered with rocks and other covering materials.
Pipes of this type often comprise several layers for providing the pipe with a sufficient strength during use. The present invention relates in particular to a system comprising a pipe comprising a flow channel and an annular fluid cavity surrounding the flow channel. Such an annular fluid cavity is often equipped with a gas drainage valve in order to ensure that the pressure within the annular cavity does not exceed a certain selected pressure. Such a pipe with a gas drainage valve is for example described in U.S. Pat. No. 7,296,480. During use fluid components such as gasses from the medium immediately surrounding the pipe (e.g. sea water) and/or from the fluid transported in the flow channel are penetrating into the annulus. Many of these fluid components are very aggressive and in order to estimate the lifetime of the pipe it is desirable to know the qualitative and/or quantitative amount of such fluid components. Other less aggressive fluid components in the annulus can also be used in the analysis of the general state of the pipe as well as in the analysis of the fluid transported in the flow channel and the pressure/temperature conditions along the pipe. Furthermore a qualitative and/or quantitative determination of one or more fluid components in the annulus may be used to determine failure of the pipe in general such a local leakage or other fractures of one or more layers of the pipe.
The quantitative amount may be determined as the absolute quantitative amount e.g. per volume unit or it may be determined as the relative quantitative amount e.g. in relation to another fluid component, in relation to a set value, or in relation to any other value which makes sense to the skilled person.
It is therefore desirable to obtain qualitative and/or quantitative measurements of the fluid components in the annulus of the pipe, with respect to one or more selected fluid components and in particular one or more gas components.
WO 03/056313 describes a sensor system for use in the detection or measurement of at least one characteristic relating to a chemical environment in a flexible pipe. The system comprises incorporating an optical fibre along a flexible pipe—e.g. in an armouring wire; letting a gas derived from the chemical environment diffuse into the optical fibre, thereby altering the optical properties of the optical fibre; detecting and analyzing light from the optical fibre so as to determine changes in the optical properties of the optical fibre due to the in-diffusion of said gas; and deriving said at least one characteristic value representing the chemical environment from the determined changes. This system, which has never been used in practice in a flexible pipe, is however rather complicated to use as it reacts relatively slowly and it is difficult to obtain separate determination of desired fluid components. The traditional method of determining fluid components from the fluid in the annulus is still to take out samples at regular intervals and to test them in the laboratory.
The object of the invention is therefore to provide a pipe system comprising a pipe comprising a flow channel and an annular fluid cavity surrounding the flow channel and which comprises a safe and simple arrangement for obtaining qualitative and/or quantitative determinations of one or more selected fluid components in the annulus.